METHOD AND DEVICE FOR REDUCING HEAVY POLYCYCLIC AROMATIC COMPOUNDS IN HYDROCRACKING UNITS

Abstract

The invention concerns a process and a facility for reducing the concentration of heavy polycyclic aromatic compounds (HPNA) in the recycle loop of hydrocracking units, which comprises a fractionation column.

In accordance with this process, a portion of the stream present at the level of at least one plate (I) which is the supply plate or a plate located between the supply plate and said residue evacuation point, or if stripping gas is injected, between the supply plate and said stripping gas injection point, is withdrawn from the fractionation column.

A portion, preferably all, of said withdrawn stream is recycled to the hydrocracking step directly or after optional separation of the gases. The residue is purged in its entirety.

In a preferred embodiment, a portion of the stream present at the level of at least one plate (II) located between the supply plate and the plate for withdrawing the heaviest distillate fraction is also withdrawn from the column. After stripping, all or a portion of the gas is recycled to the column and the liquid is sent for hydrocracking.

Claims

1. A process for hydrocracking an oil feed comprising at least 10% by volume of compounds boiling above 340° C., comprising a hydrocracking step, optionally followed by a separation of the gases from the hydrocracked effluent, then a step for fractionation of said effluent, which separates at least one distillate and a residue, a portion of said residue being recycled to the hydrocracking step and another portion of the residue being purged, said fractionation step comprising a distillation in a column provided with plates, in which column: said at least partially vaporized effluent is supplied to the column over at least one supply plate, said distillate is withdrawn from the level of a withdrawal plate, said residue is evacuated at an evacuation point, and optionally, a stripping gas is injected at an injection point located below the supply plate, in which process a portion of the stream present at the level of at least one plate (I) which is the supply plate or a plate located between the supply plate and said residue evacuation point, or if injection gas is injected, between the supply plate and said stripping gas injection point is withdrawn from the column, all or a portion of said withdrawn stream is recycled to the hydrocracking step, and the residue is purged in its entirety.

2. The process as claimed in claim 1, in which a portion of the stream present at the level of the supply plate or of a plate located below the supply plate and close to said supply plate, preferably at the level of the plate which is closest to the supply plate, is withdrawn from the column.

3. The process as claimed in claim 1, in which said withdrawn stream is recycled to the hydrocracking step directly or after optionally separating the gases, and preferably directly.

4. The process as claimed in claim 1, in which a portion of the stream present at the level of at least one plate (II) located between the supply plate and the withdrawal plate for the heaviest distillate is withdrawn from the column.

5. The process as claimed in claim 1, in which the stream withdrawn from the level of the plate (I) or the plate (II) has a concentration of HPNA of less than 500 ppm by weight, preferably less than 350 ppm by weight.

6. The process as claimed in claim 1, in which the stream withdrawn from the level of the plate (I) or the plate (II) has a proportion of at least 70% by weight of unconverted hydrocarbons, preferably at least 80% by weight of unconverted hydrocarbons.

7. The process as claimed in claim 4, in which all or a portion, preferably all, of said stream withdrawn from said plate (II) is stripped in an external stripping step by a stripping gas, and all or a portion, preferably all, of the separated gaseous effluent is recycled to the column above the plate from which said stream has been withdrawn, and all or a portion, preferably all, of the separated liquid effluent is recycled to the hydrocracking step.

8. The process as claimed in claim 7, in which all or a portion, preferably all, of said withdrawn stream is stripped in an external stripping step using a stripping gas, and all or a portion, preferably all, of the separated gaseous effluent is recycled to the column at the level of the plate closest to the plate from which said stream has been withdrawn.

9. The process as claimed in claim 4, in which the stripping gas injected into the external stripping step is steam, preferably at a pressure in the range 0.2 to 1.5 MPa.

10. The process as claimed in claim 1, in which a stripping gas is injected into the fractionation step, the gas preferably being steam, preferably at a pressure in the range 0.2 to 1.5 MPa.

11. A facility comprising: a hydrocracking section (2) provided with an inlet line (1) for the feed and an inlet line (8) for hydrogen, optionally followed by a zone (4) for separating effluent in order to separate a gaseous fraction, followed by a fractionation section (12) comprising at least one distillation column provided with plates, said column comprising: at least one line (11) for the inflow of at least partially vaporized hydrocracked effluent onto at least one supply plate, at least one line (14) for withdrawing at least one distillate from the level of a withdrawal plate, at least one line (16) for evacuating the entirety of the residue, and optionally comprising at least one line (19) for injecting a stripping gas, the injection point being located below the supply plate, the facility further comprising: at least one line (20) for withdrawing a portion of the stream present at the level of at least one plate (I) which is the supply plate or a plate located between the supply plate and said residue evacuation point, or if injection gas is injected, between the supply plate and said stripping gas injection point, at least one line (18) for recycling all or a portion of said withdrawn stream, preferably all, to the hydrocracking step.

12. The facility as claimed in claim 11, comprising at least one line (18) for recycling said withdrawn stream in its entirety directly to the hydrocracking step.

13. The facility as claimed in claim 11, further comprising: at least one line (21) for withdrawing a portion of the stream present at the level of at least one plate located between the supply plate and the plate for withdrawing the heaviest distillate fraction, a stripper (25) external to the column, provided with an inlet line (21) for said withdrawn stream, a stripping gas injection line (26), an outlet line (22) for the gaseous fraction, an outlet line (23) for the liquid fraction, a line (22) for recycling all or a portion, preferably all, of said gaseous fraction to said column, the line (22) discharging into the column above the plate from which said stream has been withdrawn, and preferably at the level of the plate closest to the plate from which said stream has been withdrawn, a line (23) for recycling all or a portion, preferably all, of said liquid fraction to the hydrocracking step. preferably, there is no line for recycling the liquid fraction separated in the stripping step to the fractionation column.

14. The facility as claimed in claim 11, which does not comprise a line for recycling residue to the column.

Description

[0053] The purged portion refers to a portion which leaves the process.

[0054] FIG. 1 represents the prior art. The configurations 2c and 2d of FIG. 2 represent the invention. FIGS. 2c and 2d should be construed in combination with FIG. 1, and more precisely with the essential elements of FIG. 1 cited in the claims.

[0055] The principle of the invention will become apparent starting from FIG. 2c.

[0056] FIG. 1 presents a flowchart for a prior art hydrocracking process. To facilitate reading, the description of the conditions employed has been moved to a further part of the text below.

[0057] The feed (line 1) composed of hydrocarbons of oil origin and/or synthetic hydrocarbons with a mineral or biological source is mixed with hydrogen supplied via the lines 5 (recycle) and/or 6 (makeup hydrogen) via the compressor 7 and the line 8. The feed/hydrogen mixture thus formed is sent to the hydrocracking section 2. This section comprises one or more fixed bed or ebullated bed reactors.

[0058] When the hydrocracking section comprises one or more fixed bed reactors, each reactor may comprise one or more beds of catalyst carrying out hydrocracking of the hydrocarbons of the feed to form lighter hydrocarbons.

[0059] When the hydrocracking section comprises one or more ebullated bed reactors, a stream comprising liquid, solid and gas moves vertically through a reactor containing a bed of catalyst. The catalyst in the bed is maintained in a random motion in the liquid. The gross volume of the catalyst dispersed through the liquid is thus larger than the volume of catalyst when stopped. This technology has been widely described in the literature.

[0060] A mixture of liquid hydrocarbon and hydrogen is passed through the bed of particles of catalyst at a velocity such that the particles are caused to move in a random manner and thus become suspended in the liquid. Expansion of the catalytic bed in the liquid phase is controlled by the flow rate of recycle liquid in a manner such that in the equilibrium state, the major portion of catalyst does not go above a defined level in the reactor. The catalysts are in the form of extrudates or beads, preferably with a diameter in the range 0.8 mm to 6.5 mm in diameter.

[0061] In an ebullated bed process, large quantities of hydrogen gas and light hydrocarbon vapours rise through the reaction zone then in a zone which is free of catalyst. A portion of the liquid from the catalytic zone is recycled to the bottom of the reactor after separating a gaseous fraction and a portion is withdrawn from the reactor as product, usually at the top portion of the reactor.

[0062] The reactors used in an ebullated bed process are generally designed with a central vertical recycling conduit which acts as a flow tube for recycling liquid from the catalyst-free zone located above the ebullated bed of catalyst, via a recycling pump which can be used to recycle the liquid in the catalytic zone. Recycling the liquid means that both a uniform temperature can be maintained in the reactor and that the bed of catalyst can be kept in suspension.

[0063] The hydrocracking section may be preceded by or include one or more beds of hydrotreatment catalyst(s).

[0064] The effluent from the hydrocracking section 2 is sent via line 3 to a separation zone 4 in order to recover a gaseous fraction 5 on the one hand, along with a liquid fraction 9. The gaseous fraction 5 contains excess hydrogen which has not reacted in the reaction section 2. It is generally combined with fresh hydrogen arriving via the line 6 in order to be recycled as indicated below.

[0065] The liquid fraction 9 is reheated by any means 10, for example a furnace which could be associated with an exchanger (not shown), in order to at least partially vaporize it before supplying the fractionation section 12 via the line 11.

[0066] The fractionation section 12 comprises one or more distillation columns equipped with plates and contact means in order to separate various upgradable cuts (distillates) which are withdrawn by means of the lines 13 and 14, plus other optional side streams. These cuts have boiling point ranges situated, for example, in the gasoline, kerosene and gas oil ranges.

[0067] A heavier unconverted fraction (residue) is recovered from the bottom of the column (line 15a).

[0068] Stripping gas may be injected via the line 19. This line is located between the plate for supplying hydrocracked effluent (line 11) and the residue evacuation point (line 15a).

[0069] A portion of the residue may be purged via the line 16, with another portion recycled to the hydrocracking section via the lines 23 and 18 and another portion recycled to the fractionation section (line 15b).

[0070] In accordance with FIG. 1, a portion (line 15b) of the residue from the line 15a is mixed with the supply (line 9) upstream of the furnace 10 of the fractionation section and recycled as a mixture with this cut towards the fractionation section (line 11).

[0071] The purge 16 can in particular be used to eliminate at least a portion of the HPNA compounds which could accumulate in the recycle loop without this purge.

[0072] The zone E outlined in FIG. 1 defines the portion modified by the subject matter of the present invention.

[0073] FIGS. 2c and 2d present the invention.

[0074] The elements described above will not be described again here. It should be noted that the line 15b (recycle of residue to the fractionation column) is dispensed with in the invention. This is also the case for the recycle of residue to the hydrocracker.

[0075] The fractionation section 12 comprises a single fractionation column. However, the invention could be implemented with several fractionation columns and at least one column would then comprise a zone E in accordance with the invention.

[0076] In accordance with FIG. 2c, the liquid fraction 11 which has previously been at least partially vaporized is supplied to the fractionation section 12.

[0077] Preferably, a stripping gas is injected into the column (line 19). Advantageously, it is steam, preferably low pressure steam, preferably at a pressure in the range 0.2 to 1.5 MPa (0.1 MPa=1 bar). The injection point is located below the supply plate and above the residue evacuation point. It is preferably close to the point for evacuation of residue from the bottom of the column.

[0078] FIG. 2c differs from FIG. 1 primarily in that a side stream is added (line 20) at the level of one of the plates of the column. It is possible to position one or more side streams at the level of the column. Thus, a portion of the stream present at the level of at least one plate (I) is withdrawn.

[0079] In a preferred embodiment, this plate may be a supply plate. In FIG. 2c, the plate (I) shown is the supply plate.

[0080] This may also be a plate located between the supply plate and said residue evacuation point or in fact, if injection gas is injected, between the supply plate and said injection point for stripping gas. This withdrawal (line 20) is preferably at the level of a plate close to the supply plate, and preferably at the level of the plate closest to the supply plate.

[0081] The side stream (line 20) is positioned in a manner such that the withdrawn stream has a low concentration of HPNA of less than 500 ppm by weight, preferably less than 350 ppm by weight and highly preferably less than 200 ppm by weight, and most often, a large proportion of unconverted hydrocarbons in the hydrocracking section of at least 70% by weight of unconverted hydrocarbons, preferably at least 80% by weight of unconverted hydrocarbons and highly preferably at least 90% by weight of unconverted hydrocarbons.

[0082] In order to satisfy these criteria, the side stream (line 20) is preferably positioned at the level of the supply plate or in fact below the supply plate, and in the latter case, preferably at the level of the plate closest to the supply plate.

[0083] All or a portion of said withdrawn stream is recycled to the hydrocracking step. It may be recycled directly (i.e. without treatment) or after optional separation of the gases. Preferably, it is recycled directly to the hydrocracking step.

[0084] In accordance with the invention, the residue is not recycled to the column or to the hydrocracking step. It is purged in its entirety. It should also be noted that the stream withdrawn from plate (I) is not recycled to the column 12.

[0085] The reference numerals in FIGS. 1 and 2c will not be described again in respect of the description of FIG. 2d. FIG. 2d represents a preferred embodiment of the invention with the addition of a second side stream at the level of a plate (II) which is different from plate (I).

[0086] In accordance with FIG. 2d, a portion of the stream present at the level of at least one plate (II) located between the supply plate and the plate for withdrawing the heaviest distillate fraction is withdrawn from the column (line 21).

[0087] It is possible to position one or more side streams at the level of the column. This side stream (line 21) is preferably close to the supply plate. Preferably, a portion of the stream present at the level of the upper plate closest to the supply plate is withdrawn from the column.

[0088] The side stream (line 21) is positioned in a manner such that the withdrawn stream has a low concentration of HPNA of less than 500 ppm by weight, preferably less than 350 ppm by weight and highly preferably less than 200 ppm by weight, and most often, a large proportion of unconverted hydrocarbons in the hydrocracking section of at least 70% by weight of unconverted hydrocarbons, preferably at least 80% by weight of unconverted hydrocarbons and highly preferably at least 90% by weight of unconverted hydrocarbons.

[0089] In order to satisfy these criteria, the side stream (line 21) is preferably positioned at the level of the supply plate or in fact above the supply plate, and in the latter case, preferably at the level of the plate closest to the supply plate.

[0090] All or a portion of said withdrawn stream is recycled to the column after separation of the liquid.

[0091] The withdrawn stream (line 21) is stripped in an external stripping step (stripper 25) by a stripping gas (supplied via the line 26). All or a portion of the separated gaseous effluent is recycled to the column (line 22); in accordance with FIG. 2d, the gaseous effluent is recycled in its entirety.

[0092] Preferably, the gaseous effluent is recycled to the column above the plate from which the stream has been withdrawn. In addition, better performances are obtained when the gaseous effluent is recycled to the column to the level of the plate closest to the plate from which the stream has been withdrawn.

[0093] All or a portion of the liquid effluent (line 23) is recycled to the hydrocracking step. It may be recycled directly (i.e. without treatment) or after optional separation of the gases. Preferably, it is recycled directly to the hydrocracking step.

[0094] In accordance with FIG. 2d, all of the liquid effluent (line 23) is mixed with the stream (line 20) from the side stream from the plate (I) and the mixture is recycled (line 18) to the hydrocracking step.

[0095] Said lateral stripper 25 functions with injection of a stripping gas (line 26). This gas is preferably steam, preferably low pressure steam, preferably at a pressure in the range 0.2 to 1.5 MPa.

[0096] As will be demonstrated in the examples below, the embodiment of FIG. 2d results in better performances than the embodiment of FIG. 2c.

Description of the Conditions for the Hydrocracking, 2, and Separation Steps

[0097] This description refers to conventional implementation conditions which can be applied both to FIG. 1 (prior art) and to the invention (FIGS. 2c and 2d).

Feeds:

[0098] A wide variety of feeds may be treated in hydrocracking processes. In general, they contain at least 10% by volume, generally at least 20% by volume and often at least 80% by volume of compounds boiling above 340° C.

[0099] The feed may, for example, be LCO (light cycle oil—light gas oils obtained from a catalytic cracking unit), atmospheric distillates, vacuum distillates, for example gas oils obtained from straight run distillation of crude or from conversion units such as FCC, coking or visbreaking, as well as feeds originating from units for the extraction of aromatics from lubricating oil bases or obtained from solvent dewaxing of lubricating base oils, or in fact from distillates originating from processes for fixed bed or ebullated bed hydroconversion or desulphurization of AR (atmospheric residues) and/or VR (vacuum residues) and/or deasphalted oils, or the feed may in fact be a deasphalted oil, effluents from a Fischer-Tropsch unit or in fact any mixture of the feeds cited above. The above list is not limiting.

[0100] In general, the feeds have a T5 boiling point of more than 150° C. (i.e. 95% of the compounds present in the feed have a boiling point of more than 150° C.). In the case of gas oil, the T5 point is generally approximately 150° C. In the case of VGO, the T5 is generally more than 340° C., or even more than 370° C. The feeds which may be used thus fall within a wide range of boiling points. This range generally extends from gas oil to VGO, encompassing all possible mixtures with other feeds, for example LCO.

[0101] The nitrogen content of the feeds treated in the hydrocracking processes is usually more than 500 ppm by weight, generally in the range 500 to 10000 ppm by weight, more generally in the range 700 to 4500 ppm by weight and still more generally in the range 800 to 4500 ppm by weight.

[0102] The sulphur content in the feeds treated in the hydrocracking processes is usually in the range 0.01% to 5% by weight, generally in the range 0.2% to 4% by weight and yet more generally in the range 0.5% to 3% by weight. The feed may optionally contain metals. The cumulative nickel and vanadium content in the feeds treated in hydrocracking processes is preferably less than 10 ppm by weight, preferably less than 5 ppm by weight and yet more preferably less than 2 ppm by weight. The asphaltenes content is generally less than 3000 ppm by weight, preferably less than 1000 ppm by weight, and yet more preferably less than 300 ppm by weight.

Guard Beds

[0103] In the case in which the feed contains compounds of the resins and/or asphaltenes type, it is advantageous to pass the feed initially over a bed of catalyst or adsorbent which differs from the hydrocracking or hydrotreatment catalyst. The catalysts or guard beds used are in the shape of spheres or extrudates. Any other shape may be used. Particular possible shapes which may be used are included in the following non-limiting list: hollow cylinders, hollow rings, Raschig rings, toothed hollow cylinders, crenelated hollow cylinders, wheels known as pentarings, multiple-holed cylinders, etc.

[0104] These catalysts may have been impregnated with a phase which may or may not be active. Preferably, the catalysts are impregnated with a hydrodehydrogenating phase. Highly preferably, the CoMo or NiMo phase is used. These catalysts may have a macroporosity.

Operating Conditions:

[0105] The operating conditions such as temperature, pressure, hydrogen recycle ratio, or hourly space velocity may vary widely as a function of the nature of the feed, the quality of the desired products and the facilities available to the refiner. The hydrocracking/hydroconversion catalyst or hydrotreatment catalyst is generally brought into contact with the feeds described above in the presence of hydrogen, at a temperature of more than 200° C., often in the range 250° C. to 480° C., advantageously in the range 320° C. to 450° C., preferably in the range 330° C. to 435° C., at a pressure of more than 1 MPa, often in the range 2 to 25 MPa, preferably in the range 3 to 20 MPa, the space velocity being in the range 0.1 to 20 h.sup.1, preferably in the range 0.1 to 6 h.sup.1, and more preferably in the range 0.2 to 3 h.sup.−1, and the quantity of hydrogen introduced being such that the volume ratio in litres of hydrogen/litres of hydrocarbon is in the range 80 to 5000 NL/L, usually in the range 100 to 3000 NL/L.

[0106] These operating conditions used in the hydrocracking processes can generally be used to obtain conversions per pass into converted products (i.e. with boiling points below the residue cut point) of more than 15%, and more preferably in the range 20% to 95%.

The Principal Aims:

[0107] The invention may be used in all hydrocrackers, namely: [0108] the maxi-naphtha hydrocracker with a residue cut point which is generally between 150° C. and 190° C., preferably between 160° C. and 190° C., and usually 170° C.-180° C., [0109] the maxi-kerosene hydrocracker with a residue cut point which is generally between 240° C. and 290° C., and usually 260° C.-280° C., [0110] the maxi-gas oil hydrocracker with a residue cut point which is generally between 340° C. and 385° C., and usually 360° C.-380° C.

EMBODIMENTS

[0111] The hydrocracking/hydroconversion processes using the catalysts in accordance with the invention cover the ranges of pressure and conversion from mild hydrocracking to high pressure hydrocracking.

[0112] The term “mild hydrocracking” means hydrocracking resulting in moderate conversions, generally below 40%, and operating at low pressures, generally between 2 MPa and 9 MPa. The hydrocracking catalyst may be used alone, in a single or in more fixed bed catalytic beds, in one or more reactors, in a “once-through” hydrocracking layout, with or without liquid recycling of the unconverted fraction, optionally in association with a hydrorefining catalyst located upstream of the hydrocracking catalyst.

[0113] The hydrocracking may be operated at high pressure (at least 10 MPa).

[0114] In a first variation, the hydrocracking may be operated in accordance with a hydrocracking layout which is known as a “two-step” layout, with intermediate separation between the two reaction zones; in a given step, the hydrocracking catalyst may be used in one or in both reactors associated or otherwise with a hydrorefining catalyst located upstream of the hydrocracking catalyst.

[0115] In a second variation, what is known as “once-through” hydrocracking may be carried out. This variation generally initially comprises intense hydrorefining which is intended to carry out intense hydrodenitrogenation and hydrodesulphurization of the feed before it is passed over the hydrocracking catalyst proper, in particular in the case in which it comprises a zeolite. This intense hydrorefining of the feed brings about only a limited conversion of this feed into lighter fractions. The conversion, which is still insufficient, must therefore be supplemented on the more active hydrocracking catalyst.

[0116] The hydrocracking section may contain one or more beds of identical or different catalysts. When the preferred products are middle distillates, basic amorphous solids are used, for example alumina or silica-aluminas or basic zeolites, optionally supplemented with at least one hydrogenating metal from group VIII and preferably also supplemented with at least one metal from group VIB. These basic zeolites are composed of silica, alumina and one or more exchangeable cations such as sodium, magnesium, calcium or rare earths.

[0117] When gasoline is the major desired product, the catalyst is generally composed of a crystalline zeolite onto which small quantities of a metal from group VIII are deposited, and also, more preferably, a metal from group VIB.

[0118] The zeolites which may be used are natural or synthetic and may, for example, be selected from X, Y or L zeolites, faujasite, mordenite, erionite or chabasite.

[0119] Hydrocracking may be carried out in just one or in more ebullated bed reactors, with or without a liquid recycle of the unconverted fraction, optionally in association with a hydrorefining catalyst located in a fixed bed or ebullated bed reactor upstream of the hydrocracking catalyst. The ebullated bed is operated with withdrawal of spent catalyst and the daily addition of fresh catalyst in order to keep the catalyst activity stable.

Liquid/Gas Separation (4):

[0120] The separator 4 separates the liquid and gas present in the effluent leaving the hydrocracking unit. Any type of separator that can carry out this separation may be used, for example a flash drum, a stripper, or even a simple distillation column.

Fractionation (12):

[0121] The fractionation section is generally constituted by one or more columns comprising a plurality of plates and/or internal packing which may preferably be operated in counter-current mode. These columns are usually steam stripped and include a reboiler in order to facilitate vaporization. It can be used to separate hydrogen sulphide (H.sub.2S) and light components (methane, ethane, propane, butane etc) from the effluents, as well as the hydrocarbon cuts with boiling points in the gasoline, kerosene and gas oil ranges along with a heavy fraction recovered from the bottom of the column, all or a portion of which may be recycled to the hydrocracking section.

EXAMPLES

Example 1: Prior Art

[0122] This example is based on the configuration of FIG. 1. Two samples from an operating industrial unit based on the configuration of FIG. 1 were analysed. The properties are recorded in Table 1 below.

[0123] It should be noted that because of the configuration, the streams 15a, 16, 18 and 23 had exactly the same properties.

[0124] The fractionation of stream 11 in the column 12 was computer simulated using the PRO/II version 8.3.3 software marketed by SimSci. The physical and analytical properties of the resulting streams were simulated and compared with the physical and analytical properties of actual samples.

[0125] The operating conditions for the column used for the simulation are recorded in Table 2 below.

[0126] Starting from the properties of the stream 11 entering the fractionation column (see Table 1), the PRO/II simulation was able to establish the properties of the stream 15 leaving the fractionation column; in particular, the HPNA distribution could be modelled.

[0127] Based on these results, the configurations of the invention were simulated. The results are disclosed below for each configuration 2c or 2d.

TABLE-US-00001 TABLE 1 Properties of the streams of the layout of FIG. 1 Streams from FIG. 1 Stream number Configuration 11 15a 18 16 Yield % by wt 100 42 39.5 2.5 Quantity of gas % by wt 64.0 10.9 10.9 10.9 oil in stream Specific gravity 0.805 0.828 0.828 0.828 HPNA Coronene ppm by wt 209 497 497 497 Dibenzo(e,ghi)- ppm by wt 33 78 78 78 perylene Naphtho[8,2,1 abc] ppm by wt 81 192 192 192 coronene Ovalene ppm by wt 57 135 135 135 Total HPNA ppm by wt 378 902 902 902 TBP, % by wt Initial boiling point ° C. 128 200 200 200 10% ° C. 200 368 368 368 50% ° C. 326 402 402 402 90% ° C. 440 477 477 477 Final boiling point ° C. 524 524 524 524 1: Specific gravity SG = ρ.sub.sample at 20° C./ρ.sub.H20 at 4° C., where ρ is the density expressed in g/cm.sup.3

TABLE-US-00002 TABLE 2 Operating conditions for the column Operating conditions for fractionation FIG. 1 Pressure, top of column barg 1.0 Pressure, bottom of column barg 1.5 Temperature, inlet feed ° C. 377 Number of theoretical plates 34 Flow rate of stripping steam kg of steam/tonne of feed 17

Example 2: Configuration 2c

[0128] Table 3 below provides the characteristics of the streams 11, 16 and 18 (identical to 20) in the configuration 2c obtained from the PRO/II simulation. The operating conditions for the column used for the simulation are recorded in Table 4:

TABLE-US-00003 TABLE 3 Properties of the streams of the layout of FIG. 2c Streams from FIG. 2c Stream number 11 18 16 Configuration inlet liquid recycle purge Yield 100 39.5 2.5 Quantity of gas oil in stream 64.0 16.1 11.1 Specific gravity 0.805 0.8275 0.8284 HPNA Coronene 209 420 2153 Dibenzo(e,ghi)perylene 33 91 313 Naphtho[8,2,1 abc] coronene 81 121 873 Ovalene 57 76 623 Total HPNA 378 707 3962 TBP, % by wt Initial boiling point 128 89 207 10% 200 363 368 50% 326 399 402 90% 440 475 478 Final boiling point 524 524 524 1: Specific gravity SG = ρ.sub.sample at 20° C./ρ.sub.H20 at 4° C., where ρ is the density expressed in g/cm.sup.3

TABLE-US-00004 TABLE 4 Operating conditions for the column Operating conditions for fractionation FIG. 2c Pressure, top of column barg 1.0 Pressure, bottom of column barg 1.5 Temperature, inlet feed ° C. 377 Number of theoretical plates 34 Flow rate of stripping steam kg of steam/tonne of feed 17

[0129] Compared with the configuration of FIG. 1, the configuration 2a can be used to maximize the quantity of HPNA (3962 ppm by weight compared with 902 ppm by weight in configuration 1) in the unconverted fraction which was purged via the line 16. At the same time, the quantity of HPNA was minimized in the stream which returns to the reaction section via the line 18 (707 ppm by weight compared with 902 ppm by weight in configuration 1, which reduced the quantity of HPNA by 21.6%.

[0130] In addition, the proportion of refractory and poisonous heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) compared with the total quantity of HPNA in the stream returning to the reaction section was much lower for the configuration 2c (27.8%) than for the configuration 1 (36.3%). This indicates that not only was there less total HPNA in the stream returning to the reaction section via the line 18, but also that the proportion of refractory and poisonous heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) was lower.

Example 5: Configuration 2d

[0131] Table 5 below provides the characteristics obtained from the PRO/II simulation for the streams 11, 16 and 18 in the configuration 2d. The operating conditions for the column used for the simulation are recorded in Table 6:

TABLE-US-00005 TABLE 5 Properties of the streams of the layout of FIG. 2d Streams from FIG. 2d Stream number 11 18 16 Configuration inlet liquid recycle purge Yield 100 39.5 2.5 Quantity of gas oil in stream 64.0 6.8 4.0 Specific gravity 0.805 0.8273 0.8338 HPNA Coronene 209 405 2682 Dibenzo(e,ghi)perylene 33 106 379 Naphtho[8,2,1 abc] coronene 81 87 1106 Ovalene 57 46 792 Total HPNA 378 644 4959 TBP, % by wt Initial boiling point 128 78 298 10% 200 388 388 50% 326 399 442 90% 440 474 516 Final boiling point 524 524 524 1: Specific gravity SG = ρ.sub.sample at 20° C./ρ.sub.H20 at 4° C., where ρ is the density expressed in g/cm.sup.3

TABLE-US-00006 TABLE 6 Operating conditions for the column Operating conditions for fractionation FIG. 2d Pressure, top of column barg 1.0 Pressure, bottom of column barg 1.5 Temperature, inlet feed ° C. 377 Number of theoretical plates 34 Flow rate of stripping steam kg of steam/tonne of feed 17 Operating conditions for side stripper Pressure, top of column barg 1.4 Pressure, bottom of column barg 1.5 Number of theoretical plates 6 Flow rate of stripping steam kg of steam/tonne of feed 28

[0132] Compared with the configuration of FIG. 1, configuration 2d can be used to maximize the quantity of HPNA (4959 ppm by weight compared with 902 ppm by weight for configuration 1) in the unconverted fraction which was purged via the line 16.

[0133] At the same time, the quantity of HPNA was minimized in the stream leaving the reaction section via the line 18 (644 ppm by weight compared with 902 ppm by weight for configuration 1), which reduced the quantity of HPNA by 28.6%.

[0134] In addition, the proportion of the most refractory and poisonous heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) compared with the total quantity of HPNA in the stream 18 returning to the reaction section was much lower for configuration 2d (20.7%) than for configuration 1 (36.3%). This indicates that not only was there less total HPNA in the stream returning to the reaction section via the line 18, but also that the proportion of heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) was lower.

[0135] This configuration could also minimize the quantity of gas oil returned to the reaction section via the line 18 because the quantity of gas oil returned to the reaction section was only 6.8% by weight compared with 10.9% by weight in configuration 1.